1. Field of the Invention
The present invention relates to an electronic source apparatus. More specifically, the present invention relates to a sound source synthesizer suitable for producing sound in an electronic game machine such as a television game machine using a video monitor.
2. Description of the Prior Art
The pertinent prior art relating to the present invention is disclosed in Japanese Patent Publication No. 33525/1979 issued Oct. 22, 1979. The prior art is a musical sound closely resembling the tone of musical instruments by using frequency modulation (FM) to induce time variation to the spectrum of the sound according to the kinds of musical instruments, such as brass or wood.
The prior art may be utilized efficaciously in keyed instruments to produce the musical sound equivalent to the various kinds of musical instruments.
In the electronic game machine such as the television game machine, it is necessary to produce sound to enhance the game and to interest the players. In one example of sound a constantly changing sound, hereinafter referred to as a sweep sound, is produced by continuously increasing or reducing the frequency.
However, in the above mentioned prior art, such a sweep sound could not be produced, for the following reason.
The musical sound signal (e) in the prior art may be given by the following equation: EQU e=A sin [w.sub.c t+I (t) sin w.sub.m t]
In the above equation, if the term I(t) sin w.sub.m t is integrated, the result will be ##EQU1## which oscillates within the range of I(t) as the maximum and -I(t) as the minimum, resulting in a mean value of "0", thus a mean value of the musical sound signal (e) may be given as, EQU e=A sin w.sub.c t.
Thus, in the prior art, the mean frequency is constant and the "sweep sound" described above can not be produced.